A security manager for the Saks Fifth Avenue store in Beverly Hills said on the witness stand Monday that when he detained actress Winona Ryder last December on suspicion of shoplifting, she told him she was preparing for an acting role.

Wearing a simple black dress with pink embroidered flowers on the collar and a black headband, Ryder, whose parents sat in the spectator seats of the courtroom, kept her cool during most of Monday's proceedings, reports PEOPLE.

She talked to her attorney, fiddled with the cap of a water bottle, took notes and shook her head a lot. She also started to cry after the security guard, Kenneth Evans, testified that he had asked authorities if there was any way to file charges on another day in order to spare Ryder the spectacle of being hauled away by police.

Ryder, whose 31st birthday is Tuesday, is being tried in Beverly Hills Superior Court on charges of burglary, grand theft and vandalism. Authorities claim she swiped nearly $6,000 in merchandise from Saks. She has pleaded not guilty to all three charges.

In his testimony Monday, Evans described Ryder as "polite and apologetic" when she was stopped leaving the store with some 20 items of designer clothes, handbags and accessories. He alleged that actions inside Saks were captured on a 90-minute store surveillance videotape that was shown to the jury.

When police were alerted, Ryder "immediately stood up and took hold of my hand and apologized for what had happened," Evans said, adding that she then told him that "my director directed me to shoplift in preparation for a role I am preparing."

It was not clear what role or to which director Ryder was referring, Reuters reports.